registry: add pyenv

Fix #274 

Add dpaste

Supersedes #430

* omf/templates: use ones name as copyright holder

* omf/templates: use ones github username in URLs

* omf/templates: use current yeat in copyright notice

* assert fish version before asserting commands

- fix #398 so that the correct error message is displayed

* move assert_cmds to after fish and git checks

- remove fish and git from assert_cmds, these checks are already done in their specific assert_ functions making them redundant in assert_cmds
- move function assert_cmds to match execution order

* Add eden to omf theme registry.

* Add theme-eden repo to oh-my-fish.

Refactor the installer to be more maintainable and interactive. The installer is now more robust and safe in its operation, and asks interactive questions for choices that the user must resolve. Setting up confuguration is also now changed to take advantage of Fish 2.3 features and does not mess with user's configuration files without permission.

- Make installer smarter and use uninstaller in destroy
- Better handling and checking for offline installs
- `omf destroy` simply uses `install --uninstall`
- Final warning message is no longer displayed when user intentionally aborts install
- Backups are restored during uninstall
- Update the README to detail the new ways to install OMF

Fixes #377